September 8, 2010

The Eyes of the Mantis Shrimp are Awesome

If there’s one thing readers have come to expect from Trouble Thinking, it’s facts about sea creatures. If there’s two things readers have come to expect from Trouble Thinking, it’s facts about the Mantis Shrimp.

Fact 1: 100% Excellent.

That colorful little dude up there has some kind of awesome abilities, like claws that move so fast they cause cavitation, a shockwave through the water, and hit prey twice with a single strike.

But we can address that in the many Mantis Shrimp Facts posts to follow, what I’m concerned with at the moment is their amazing eyesight. Mantis Shrimp, for some reason, have probably the best eyesight in the entire animal kingdom. You know how your dog always laughs at you when you try to figure out where that dog-urine smell is coming from? Mantis Shrimp see us looking at things with our big squishy useless eyeballs and they just quietly fluoresce to each other. “Come on, let’s leave them be” they say “they can’t even see us laughing at them. This is just sad.”

The Mantis Shrimp Eyeball looks like this

It's okay to stare.

“Wait” you’re saying to yourself “that’s not AN eyeball, that’s three eyeballs.” Congratulations, it’s just dawning on you.

You know, of course, that human vision is binocular. That is, having two eyeballs means we see things from two angles, which allows us to perceive depth. Close one eye, and suddenly the world goes flat. Each of the eyes of a Mantis Shrimp has three visual centers, making them the only animal in the world to have trinocular vision, and they have it even if they close one eye.

So what does this triple-eye see? Well, the top and bottom hemispheres are pretty normal. They perceive depth, obviously, but also the general form and motion of objects. Being able to do that with a single eye would be enough to make them interesting, but it’s that middle band of photoreceptors that gives them superpowers.

The band consists of 6 rows. Rows 1-4 are specialized for hyperspectral color vision. So whereas humans see a small portion of the electromagnetic spectrum as visible light

Mantis Shrimps see all the way from the bottom of Infra-Red to the top of Ultra-Violet. If I understand this right, it means that they possess the ability to see thoughts and read fate-lines. Not only can the Mantis Shrimp see deep into the hyperspectrum, rows 1-4 actually come with built-in color filters that can be adjusted for local conditions. The 5th and 6th rows of the midband can detect polarized light.

Polarized light is probably most famous to humans for being the thing we use sunglasses to blot out, and the thing that makes the new better 3D theater stuff possible. Basically, light waves normally just sort of vibrate at random, but polarized light waves only go one direction. So when you filter out glare with sunglasses, you’re basically putting on those stupid looking Kanye West glasses, but with a much tinier filter. The horizontal slats won’t allow in vertically polarized light.

Light can also be circularly polarized, spinning around like a helix on it’s way to your eyeballs. Some animals can naturally see non-circular polarized light. No animal on Earth can see circular polarized light, except for humans and Mantis Shrimp. Humans because we’ve developed advanced technology that allows us to investigate the phenomenon, and Mantis Shrimp because they are awesome. In fact, some species of Mantis Shrimp may be able to detect polarization across the spectrum with greater fidelity than any human device.

It is speculated that part of the reason the Mantis Shrimp possess this ability is that it allows them to communicate in secret, using circularly polarized light to transmit messages that their eternal foe the Dread Cuttlefish cannot see.

Although this midband sees a comparatively small portion of the visual field at any given time, the eyes of the Mantis Shrimp are located on two incredibly mobile stalks. Even for a crustacean, they have the ability to independently manipulate the position of their eyes to an amazing degree, allowing them to scan a huge amount of the visual field with their higher-power midband. And to keep this all in perspective, their visual processing seems to be done in a parallel manner from multiple retinal data streams in order to reduce the time required for analysis.

So, essentially, the Mantis Shrimp is the greatest natural eye in the world, and may still outperform our own artificial creations. The question is: why? Though there are several practical benefits, like the ability to secretly communicate with polarized light, that doesn’t do everything to explain this natural marvel. One possible and I think excellent explanation is that their eyes evolved not unlike the Peacock’s tail. Mating for Mantis Shrimps involves flourescing, making their bodies all colorful under various spectra. In order to better appreciate this display under adverse conditions, the eye just kept getting more and more crazy excellent. So instead of getting a big silly tail, the Mantis Shrimp got the world’s most perfect eye.

And they're alllll natural baby.

Peacocks kind of got shafted in the mating adaptation department.

But really, we all got burned when in the eye department. Think about that the next time you watch a sunset. Think about the fact that if the Mantis Shrimp were there he’d be appreciating it so much more than you. Think about the fact that compared to him you’re basically blind. Think about it and weep with your weak little eyeballs.

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I think you may be overstating the wavelength range- their multi-spectral range reaches from UV to IR, but not necessarily to the extreme ends of those bands. Water is nearly opaque beyond 800 nm- see http://en.wikipedia.org/wiki/File:Water_absorption_spectrum.png for a spectral plot. The mantis shrimp eyes probably don’t work outside of the range of 200-800 nm, but still damn impressive.

So it appears that they have wide angle monochromatic vision at modest resolution, along with a central vision area with remarkable color and polarization discrimination. The total number of elements is around 10,000 so the pixel count is very modest.